Recently, many wireless communication systems employ a technique that performs a digital processing in order to realize a highly efficient transmission. In the wireless communication system that employs a multi-value phase modulation technique, it may be important for a transmission end to linearize an amplifying characteristic of a power amplifier in order to suppress a non-linear distortion, and to reduce an adjacent channel leakage.
In addition, the non-linear distortion occurs when the linearity of the power amplifier is relatively poor. For this reason, it may be required to compensate for the non-linear distortion in a case in which the power efficiency is to be improved while using the power amplifier having a relatively poor linearity.
FIG. 1 is a block diagram illustrating an example of a transmission apparatus. In the transmission apparatus illustrated in FIG. 1, a transmitting signal generating unit 1 generates a digital data sequence, and inputs the serial digital data sequence to a S/P (Serial-to-Parallel) converter 2. The S/P converter 2 converts the digital data sequence into signals of two systems, namely, a phase component signal and a quadrature component signal, by alternatively allocating the digital data sequence in units of 1 bit. The phase component signal may be referred to as an I signal (In-Phase component), and the quadrature component may be referred to as a Q signal (Quadrature component). The I signal and the Q signal are input to a D/A (Digital-to-Analog) converter 3. The D/A converter 3 converts the I signal and the Q signal into a baseband signal, and inputs the baseband signal to a quadrature modulator 4.
The quadrature modulator 4 multiplies a reference carrier wave and a phase-shifted carrier wave obtained by phase-shifting the reference carrier wave by 90° to the I signal and the Q signal that are converted into the analog baseband signal, respectively. In addition, the quadrature modulator 4 carries out a quadrature transformation by adding the signal that is obtained by multiplying the reference carrier wave to the analog baseband signal, and the signal that is obtained by multiplying the phase-shifted carrier wave to the analog baseband signal. An output quadrature modulated signal of the quadrature modulator 4 is input to a frequency converter 5.
The frequency converter mixes the quadrature modulated signal and a local oscillation signal in order to convert the quadrature modulated signal into a radio frequency signal that is input to a power amplifier 6. The power amplifier 6 amplifies the radio frequency signal, and the radio frequency signal is transmitted to air via an antenna 7.
In a mobile communication system such as a W-CDMA (Wideband Code Division Multiple Access), the transmission power may be 10 mW to several tens of W and relatively high.
FIG. 2 is a diagram illustrating an example of an input and output characteristic of the power amplifier 6. In FIG. 2, the abscissa indicates the power (dB) of the input signal (or input power) of the power amplifier 6, and the ordinate indicates the power (dB) of the output signal (or output power) of the power amplifier 6.
FIG. 2 illustrates an example in which the input and output characteristic of the power amplifier 6 includes a distortion function f(p). As indicated by a dotted line in FIG. 2, the input power and the output power are in a non-linear relationship. The non-linear distortion occurs when the relationship of the input power and the output power is non-linear.
FIG. 3 is a diagram illustrating an example of a frequency spectrum in a vicinity of a transmission frequency fo. In FIG. 3, the abscissa indicates the frequency. In FIG. 3, a characteristic “a” before the non-linear distortion occurs is indicated by a dotted line, and a characteristic “b” after the non-linear distortion occurs is indicated by a solid line. In other words, the characteristic “a” represents an ideal characteristic for a case in which no non-linear distortion occurs.
Side lobes of the characteristic “b” are raised compared to those of the characteristic “a”. When the side lobes are generated, an adjacent channel leakage of the transmission wave may occur. The adjacent channel leakage of the transmission wave may cause an adjacent channel interference. In other words, as the non-linear distortion illustrated in FIG. 3 becomes larger, the power of the adjacent channel leakage of the transmission wave may become higher as illustrated in FIG. 3.
The power of the adjacent channel leakage of the transmission wave (or leakage power) may be represented by an ACPR (Adjacent Channel Power Ratio). The ACPR may represent a ratio of the power of a target channel to the power of the adjacent channel leakage. The power of the target channel may be represented by an area of a spectrum between A and A′ in FIG. 3. The power of the adjacent channel leakage may be represented by an area of a spectrum between B and B′ in FIG. 3. The leakage power may act as noise with respect to other channels, and may deteriorate the communication quality of the other channels. Hence, strict restrictions may be prescribed with respect to the leakage power.
The leakage power becomes low in a linear region I of the power amplifier illustrated 6 in FIG. 2, and becomes high in a non-linear region II of the power amplifier 6 illustrated in FIG. 2.
As the input power becomes higher, the input and output characteristic of the power amplifier 6 makes a transition from the linear region I to the non-linear region II, and thus, the linear region I may need to be widened in order to obtain a high output from the power amplifier 6. However, in order to widen the linear region I, the power amplifier 6 may require a performance that is higher than actually required. For this reason, the cost and size of the transmission apparatus may inevitably increase.
For example, Japanese Laid-Open Patent Publications No. 2001-189685 and No. 2007-49251 propose wireless apparatuses provided with a function of compensating for a distortion in the transmission power.
In the transmission apparatus utilizing an adaptive control of DPD (Digital Pre-Distortion) as the digital non-linear distortion compensation technique, the carrier wave obtained by the quadrature modulation using the modulated signal is fed back and detected. In the transmission apparatus, the amplitude of the modulated signal (or transmitting baseband signal) and the amplitude of the fed back signal (or feedback baseband signal) are digitally converted and compared, and a distortion compensation coefficient is updated in real-time based on a result of the comparison. In this type of transmission apparatus, a LUT (Look-Up Table) is used to store the distortion compensation coefficient.
In the power amplifier, the relationship between the input power and the output power becomes non-linear in a saturation region. In other words, the distortion is more easily generated when the input power is high. Accordingly, in a case in which an average power of the signal is rapidly increased, the input power rapidly increases and the adaptive control of the DPD may not be performed in a sufficiently short time to thereby generate spurious noise. The spurious noise may sometimes be referred to as spurious emission.
FIG. 4 is a diagram illustrating an example of a referring range of the LUT for a case in which the adaptive control of the DPD is performed. In FIG. 4, the abscissa indicates an address for specifying the distortion compensation coefficient. The address may be represented in dB, and the power may become higher as the address value becomes larger. FIG. 4 illustrates an example of the address range having address values of “0” to “100”. Of course, the address range may be less than “100”, or “101” or more.
The upper half of FIG. 4 illustrates an example of the referring range of the LUT for a case in which the input power is low. The referring range of the distortion compensation coefficient in the LUT is updated depending on the input power. More particularly, the referring range of the distortion compensation coefficient in the LUT increases or decreases depending on the increase or decrease in the input power. When the input power is low, the range of the distortion compensation coefficient in the LUT, that is updated depending on the input power, may follow the increase or decrease in the input power, and the adaptive control of the DPD may be performed. The upper half of FIG. 4 illustrates an example in which the range of the input power and the referring range of the distortion compensation coefficient in the updated LUT are approximately the same.
The lower half of FIG. 4 illustrates an example of the referring range of the LUT for a case in which the input power is high. The referring range of the distortion compensation coefficient in the LUT is updated depending on the input power. However, when the input power is high, the referring range of the distortion compensation coefficient in the LUT is unable to follow the increase or decrease in the input power, and a range may be generated in which the adaptive control of the DPD is impossible, because the distortion of the output power becomes large with respect to the input power. When the distortion compensation coefficient in the range that cannot be referred to when performing the adaptive control of the DPD is specified, the power may rapidly increase and become high to generate the spurious noise. The spurious noise may also be referred to as interference wave. The lower half of FIG. 4 illustrates an example in which the referring range of the distortion compensation coefficient in the LUT is narrower than the range of the input power.
The so-called ramp-up control is one method of reducing the generation of the spurious noise. In the ramp-up control, the output power is gradually increased from a low output power to a desired output power. The interference wave is generated in steps because the output power is gradually increased, however, the adaptive control of the DPD may be performed within a sufficiently short time. By gradually increasing the output power, the spurious noise that is generated when the output power is varied to the desired output power may be reduced.
FIG. 5 is a diagram illustrating an example of the referring range of the LUT for a case in which the ramp-up control is performed. In FIG. 5, the abscissa indicates the address for specifying the distortion compensation coefficient. FIG. 5 illustrates a relationship between the range of the input power for the case in which the input power is gradually increased, and the referring range of the distortion compensation coefficient in the LUT. FIG. 5 illustrates an example of the address range having address values of “0” to “100”. Of course, the address range may be less than “100”, or “101” or more.
In the case in which the ramp-up control is performed, the distortion compensation coefficient in the range that cannot be referred to when performing the adaptive control of the DPD may be specified. When the distortion compensation coefficient in the range that cannot be referred to when performing the adaptive control of the DPD is specified, the power may rapidly increase to generate the spurious noise. Particularly when referring to the saturation range of the power amplifier 6, a difference between the initial LUT before the adaptive control and the LUT after the adaptive control may be large, which in turn may increase the spurious noise. For this reason, it may be desirable to reduce the generation of the spurious noise that cannot be prevented solely by the ramp-up control.